Method and device for modulating an active load

ABSTRACT

In a general aspect, a method can include producing, by an inductive antenna circuit, a first periodic signal that is based on an alternating external magnetic field; producing, by an oscillator circuit, a second periodic signal that is based on the first periodic signal; and transmitting, in correspondence with a data-carrying modulation signal, a sequence of data bits. The transmitting can include sequentially and repetitively: applying, with the oscillator circuit operating in a free oscillation mode, the second periodic signal to the inductive antenna circuit; and inhibiting, with the oscillator circuit operating in a synchronous oscillation mode, application of the second periodic signal to the inductive antenna circuit. The synchronous oscillation mode of the oscillator circuit can cause the second periodic signal to be synchronized to the first periodic signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/433,884, filed Feb. 15, 2017, which is a continuation of U.S.application Ser. No. 15/144,034, filed May 2, 2016, which is acontinuation of U.S. application Ser. No. 13/976,527, filed Jun. 27,2013, now U.S. Pat. No. 9,331,748, which is a U.S. National PhaseApplication of PCT Application No. PCT/FR2011/052183, filed Sep. 21,2011, and which claims priority to French Application No. 1003754, filedSep. 21, 2010, the disclosures of which are all incorporated herein byreference in their entireties.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to a method for sending databy inductive coupling, including receiving an antenna signal by way ofan inductive antenna circuit in the presence of an alternating externalmagnetic field, extracting from the antenna signal a first periodicsignal, producing a second periodic signal by way of a synchronousoscillator having a synchronization input receiving the first periodicsignal, placing the oscillator in a free oscillation mode and applyingto the antenna circuit bursts of the second periodic signal to generatean active load modulation magnetic field.

Embodiments of the present invention also relate to a device for sendingand receiving data designed to implement this method.

Generally speaking, the present invention relates to inductive couplingcommunication techniques, also known as “Near Field Communications” orNFC. A communication by inductive coupling generally involves aso-called passive device and a so-called active device. The two devicesare equipped with an antenna coil. The active device emits a magneticfield oscillating for example at 13.56 MHz, and sends data to thepassive device by modulating the magnetic field. This magnetic field isdesignated below “external magnetic field”. The passive device sendsdata to the active device by load modulation.

The load modulation may be passive or active. Passive load modulationinvolves modifying the impedance of the antenna coil of the passivedevice at the rate of a data-carrying load modulation signal. Thisimpedance modulation affects the impedance of the antenna coil of theactive device, by inductive coupling. The active device may thereforeextract from its antenna signal the load modulation signal used by thepassive device, and deduce therefrom the data that the passive devicesends it.

Active load modulation involves emitting bursts of alternating magneticfield, at the rate of the data-carrying modulation signal. The bursts ofmagnetic field are perceived by the active device as a passive loadmodulation. This technique was proposed by the applicant in EuropeanPatent EP 1 327 222 (U.S. Pat. No. 7,098,770B2), (see FIGS. 4A to 4E,page 8 table 4, paragraph [0074].

Compared to passive load modulation, active load modulation offers agreater communication distance and/or better data transmission in ahostile environment, for example an environment disturbed by metallicmasses generating Foucault (Eddy) currents. However, active loadmodulation requires circuitry for driving the antenna coil and thus acurrent source, but consumes much less current than a continuousemission of magnetic field.

An active load modulation device cannot therefore be purely passive interms of power supply (a purely passive device being electricallypowered by the magnetic field emitted by the active device) but isnevertheless considered “passive” in that it does not emit the externalmagnetic field necessary for the communication.

To obtain a maximum communication distance, the active load modulationalso requires that the load modulation magnetic field be in phase withthe external magnetic field emitted by the active device. Phaserotations between the active load modulation magnetic field and theexternal magnetic field may cause undesirable fluctuations in thecommunication distance.

European Patent EP 1 801 741 describes an active load modulation NFCdevice using a phase locked loop to control the phase of the loadmodulation magnetic field (see e.g., FIG. 19). The phase locked loopincludes a Voltage-Controlled Oscillator (VCO), a phase comparator, anda low-pass filter supplying a control voltage to the VCO. The phasecomparator receives, as a reference frequency, a first periodic signalextracted from the antenna signal induced by the external magneticfield. The phase locked loop supplies a second periodic signal the phaseof which is set on that of the first periodic signal. In data send mode,bursts of the second periodic signal are applied to the antenna circuitto generate the bursts of magnetic field.

When the device switches into the data send mode, the first periodicsignal is no longer applied to the phase comparator and a sample holdcircuit HLD maintains the control voltage applied to the VCO. The phaselocked loop thus switches from a synchronous operating mode into a freeoscillation mode and remains in this operating mode until the end of thedata sending.

If it is desirable for the bursts of magnetic field to be in phase withthe external magnetic field, the phase locked loop must have a veryslight phase shift over the entire duration of the data send mode, whichis at least equal to the duration of sending a data frame. In practice,the maximal phase shift tolerated over this period is generally in theorder of ¼ of the period of the magnetic field oscillating at 13.56 MHz.

As an example, an ISO 14443-A frame has a duration in the order of 25.6ms. As the frequency of the periodic signal is 13.56 MHz, the phaseshift of the phase locked loop in free oscillation mode shouldpreferably not be more than 18 ns, i.e. ¼ of the period of the magneticfield oscillating at 13.56 MHz.

Now, obtaining a stability greater than 18 ns over a duration of 25.6 msmeans that the phase locked loop must offer extremely high precision, inthe order of 0.7 ppm ((18×10⁻⁹/25.6×10⁻³)*10⁶). Such precision requiresvery high quality and expensive circuitry.

It may thus be desired to provide a way of providing bursts of magneticfield having a small phase shift relative to the external magneticfield, without using extremely precise and expensive circuitry.

BRIEF SUMMARY OF THE INVENTION

To this end, the present invention proposes using a synchronousoscillator and resynchronizing the oscillator on the external magneticfield before each application to the antenna circuit of a burst of aperiodic signal supplied by the oscillator.

More particularly, some embodiments of the present invention relate to amethod for sending data by inductive coupling, including applying analternating external magnetic field to an inductive antenna circuittuned to a carrier frequency of the external magnetic field; extractingan antenna signal from the antenna circuit; extracting from the antennasignal a first periodic signal;

producing a second periodic signal by way of a synchronous oscillatorhaving a synchronous oscillation mode set in phase with the firstperiodic signal and a free oscillation mode; placing the oscillator inthe free oscillation mode; and applying a burst of the second periodicsignal to the antenna circuit. The method includes, after applying aburst of the second periodic signal to the antenna circuit, anattenuation step of modifying the impedance of the antenna circuit toattenuate the amplitude of the antenna signal; a restoration step ofceasing to modify the impedance of the antenna circuit, to restore theamplitude of the antenna signal; and a resynchronization step of placingthe oscillator in the synchronous oscillation mode before applying a newburst of the second periodic signal to the antenna circuit.

According to one embodiment, the method includes: supplying adata-carrying modulation signal, applying a burst of the second periodicsignal to the antenna circuit when the modulation signal has a firstlogic value, and inhibiting the application of the first periodic signalto a synchronization input of the oscillator during the attenuation,restoration and synchronization periods.

According to one embodiment, the method includes inhibiting theapplication of the first periodic signal to the synchronization input ofthe oscillator by way of a masking signal.

According to one embodiment, the method includes phase shifting thesecond periodic signal relative to the antenna signal.

Some embodiments of the present invention also relate to a device forsending and receiving data by inductive coupling including an inductiveantenna circuit in which an antenna signal appears in the presence of analternating external magnetic field; means for extracting a firstperiodic signal from the antenna signal; a synchronous oscillator havinga synchronization input receiving the first periodic signal, supplying asecond periodic signal, the oscillator having a synchronous oscillationmode set in phase with the first periodic signal and a free oscillationmode; and an active load modulation circuit configured to apply burstsof the second periodic signal to the antenna circuit and to generate anactive load modulation magnetic field. The device also includes meansfor modifying the impedance of the antenna circuit and is configured toplace the oscillator in the free oscillation mode; apply to the antennacircuit a burst of the second periodic signal; modify the impedance ofthe antenna circuit to attenuate the amplitude of the antenna signal;cease modifying the impedance of the antenna circuit, to restore theamplitude of the antenna signal; and place the oscillator in thesynchronous oscillation mode before applying a new burst of the secondperiodic signal to the antenna circuit.

According to one embodiment, the device is configured to supply adata-carrying modulation signal; apply a burst of the second periodicsignal to the antenna circuit when the modulation signal has a firstlogic value; and inhibit the application of the first periodic signal toa synchronization input of the oscillator during the attenuation,restoration and synchronization periods.

According to one embodiment, the device is configured to inhibit theapplication of the first periodic signal to the synchronization input ofthe oscillator by way of a masking signal.

According to one embodiment, the means for modifying the impedance ofthe antenna circuit includes a switch, the closing of which modifies theimpedance of the antenna circuit.

According to one embodiment, the means for modifying the impedance ofthe antenna circuit includes a timer to determine a duration duringwhich the impedance of the antenna circuit is modified.

According to one embodiment, the synchronous oscillator is of theastable oscillator type having a specific self-oscillation frequencydetermined by components of the oscillator.

According to one embodiment, the synchronous oscillator is of thedigital type and is configured, in the synchronous oscillation mode, tocopy at output the period of the periodic signal applied to thesynchronization input, and, in the free oscillation mode, toreconstitute at output the frequency received at the synchronizationinput during the synchronous oscillation mode.

According to one embodiment, the synchronous oscillator includes a phaselocked loop including: a phase comparator supplying a phase signal; anactive low-pass filter receiving the phase signal and supplying acontrol voltage; a voltage-controlled oscillator receiving the controlvoltage and supplying the second periodic signal; and means forinhibiting the phase comparator when switching into the free oscillationmode and for maintaining the value of the control voltage at the inputof the voltage-controlled oscillator.

According to one embodiment, the device includes means for phaseshifting the second periodic signal relative to the antenna signal.

Some embodiments of the present invention also relate to a devicecomprising a device for sending and receiving data according to thepresent invention, and at least one host processor supplying the devicewith data to be sent.

In one embodiment, the device is mounted in or on to a portable medium.

Some embodiments of the present invention also relate to a smart card,including at least one processor and a device for sending and receivingdata according to the present invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

Some embodiments of a method and a device according to the presentinvention will be described below in relation with, but not limited to,the following figures in which:

In the drawings:

FIG. 1 represents an NFC device for sending/receiving data according toan embodiment of the present invention including a synchronousoscillator;

FIGS. 2A to 2E show signals occurring in the operation of the device inFIG. 1 during the sending of data by active load modulation;

FIGS. 3A to 3D show other signals occurring in the operation of thedevice in FIG. 1 during the sending of data by active load modulation;

FIG. 4 shows a phase error between a first periodic signal and a secondperiodic signal used by the device in FIG. 1;

FIG. 5A shows the shape of an antenna signal of the device in FIG. 1upon receiving and sending data;

FIGS. 5B to 5E show signals associated with the antenna signal;

FIG. 6 is an expanded view of a part of FIG. 5A;

FIG. 7 represents a first example of an embodiment of a synchronousoscillator;

FIG. 8 represents a second example of an embodiment of a synchronousoscillator;

FIG. 9 represents an alternative embodiment of a part of the device inFIG. 1;

FIG. 10 represents another alternative embodiment of the device in FIG.1;

FIG. 11 shows an example of a portable device including a device forsending/receiving data according to the present invention; and

FIGS. 12A, 12B are top and bottom views of another example of a portabledevice including a device for sending/receiving data according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 represents a device ND1 for sending/receiving data according tothe present invention, operating by inductive coupling. The device ND1includes:

-   -   a contact communication interface circuit ICT,    -   an antenna circuit ACT tuned to a carrier frequency, comprising        an antenna coil AC1 and which can comprise various other        components such as capacitors and/or self-inductances,    -   a demodulation circuit DMCT coupled to a decoding circuit DCCT,        to receive data DTr via the antenna circuit,    -   a coding circuit CCT coupled to a modulation circuit MCT, to        send data DTx via the antenna circuit,    -   a damping circuit DCT,    -   a clock circuit CKCT, and    -   a synchronous oscillator SO.

The contact communication interface circuit ICT enables the device ND1to be linked to at least one host processor HP1. The host processor HP1supplies the data DTx and receives the data DTr. The data DTx/DTr isgenerally application data from an NFC application (transaction,payment, information exchange, or the like). In one alternative, thedevice ND1 can be autonomous and include an internal processorconfigured to manage contactless applications. The internal processorthen generates the data DTx itself and processes the data DTr.

The clock circuit CKCT and the demodulation circuit DMCT receive anantenna signal AS present in the antenna circuit ACT. The antenna signalAS is for example extracted from the antenna circuit by an amplifier A1the gain of which is controlled by an automatic gain control circuitAGC. The antenna signal AS appears in the presence of an alternatingexternal magnetic field FLD1 emitted by an external device EDV of NFCreader type, equipped with an antenna coil AC2 and operating in theactive mode (continuous emission of the magnetic field). The field FLD1oscillates for example at a carrier frequency of 13.56 MHz (ISO 14443,ISO 13693, Sony Felica® standards) and the antenna circuit ACT is tunedto this carrier frequency.

The clock circuit CKCT supplies an external clock signal CKe, or “firstperiodic signal”, the frequency of which is generally identical to thecarrier frequency, i.e. 13.56 MHz in the framework of the abovementionedstandards. In one embodiment, the circuit CKCT also supplies a signalDET that has a determined value, for example 1, when the field FLD2 isdetected. The signal DET forms a carrier detection signal that may beuseful in certain embodiments of the device ND1.

The synchronous oscillator SO receives the external clock signal CKe andsupplies an internal clock signal CKs, or “second periodic signal”. Theoscillator SO has a synchronous oscillation mode in which the phase ofthe signal CKs is set on that of the signal CKe, and a free oscillationmode in which the signal CKs is no longer set in phase with the signalCKe.

To send data DTr to the device ND1, the external device EDV applies tothe magnetic field FLD1 a modulation, for example an amplitudemodulation, by way of a data-carrying modulation signal MS(DTr). Thesignal MS(DTr) is reflected in the antenna signal AS and is extractedtherefrom by the demodulation circuit DMCT, after removal of thecarrier. The circuit DMCT supplies the modulation signal MS(DTr) to thecircuit DCCT which decodes and supplies the data DTr to thecommunication interface circuit ICT or the internal processor of thedevice ND1, if the latter is equipped with such a processor.

The data DTx to be sent to the external device EDV is supplied to thecoding circuit CCT, which then supplies the modulation circuit MCT witha data-carrying modulation signal MS(DTx). This modulation signalMS(DTx) may be modulated with a sub-carrier derived from the carrierfrequency, for example a sub-carrier of 848 kHz (ISO 14443) or of 424kHz (ISO 15693), or simply be baseband coded without any sub-carrier(Sony Felica®). To generate the sub-carrier, the circuit MCT receivesthe internal clock signal CKs supplied by the synchronous oscillator.

The modulation circuit MCT is here an active load modulation circuitimplementing the method described by the applicant in patent EuropeanPatent EP 1 327 222 (U.S. Pat. No. 7,098,770B2). The circuit MCTsupplies the antenna circuit ACT with an active load modulation signalSlm that includes bursts (wavetrains) of the internal clock signal CKs,separated by periods of non-modulation during which the signal Slm has adefault value, for example 0. The modulation circuit MCT here comprisesan AND-type logic gate G2 receiving the signals CKs and MS(DTx) andsupplying the signal Slm, and an amplifier A2 linking the output of thegate G2 and the point of application of the signal Slm to the antennacircuit ACT.

The signal Slm is here equal to 0 when the signal MS(DTx) is equal to 0,and copies the signal CKs when the signal MS(DTx) is equal to 1.Therefore, when the signal MS(DTx) is equal to 1, the antenna circuitACT receives a burst of the signal CKs and the antenna coil AC1 emits acorresponding burst of a magnetic field FLD2. The bursts of magneticfield FLD2 are detected by the external device EDV as a passive loadmodulation. The latter may thus extract, from its own antenna signal,the signal MS(DTx) to deduce therefrom the data DTx sent by the deviceND1.

The synchronous oscillator SO includes an oscillator circuit OSC1 havinga synchronization input linked to the output of a masking circuit G1,here an AND gate. The gate G1 receives at its inputs the external clocksignal CKe and a masking signal MSK. The signal MSK is supplied by amask generator MG from the modulation signal MS(DTx). The signal MSK hasa masking value, here 0, and a transparent value, here 1. The oscillatorcircuit OSC1 synchronizes itself on the external clock signal CKe whenthe latter is not masked (MSK=1) at its synchronization input, andoperates in free oscillation mode when the signal CKe is masked (MSK=0).The masking signal has the masking value during the emission of a burstof magnetic field FLD2, and is maintained at this value during asupplementary lapse of time following the emission of the burst.

The damping circuit DCT is designed to modify the impedance of theantenna circuit so as to attenuate the antenna signal AS after theemission of a burst of magnetic field FLD2. Indeed, where the qualityfactor Q of the antenna circuit is high, the antenna circuit maycontinue to resonate by self-oscillation effect. This self-oscillationphenomenon generates an antenna signal of amplitude greater than the onegenerated by the alternating external magnetic field FLD1. Such aself-oscillation signal can disturb a step of resynchronizing theoscillator that will be described below as it is not in phase with theexternal magnetic field FLD1.

In the embodiment represented in FIG. 1, the damping circuit DCTincludes a timer, produced here in the form of a counter CP1, and aswitch CSW driven by the counter CP1. The counter CPT receives at inputthe modulation signal MS(DTx) and the internal clock signal CKs andsupplies the switch CSW with a damping signal DS. A terminal of theswitch CSW is linked directly or indirectly (for example through aresistor) to a point of the antenna circuit ACT, for example a terminalof the antenna coil AC1 or any other damping point of the antennasignal. The other terminal of the switch CSW is linked to the ground.The switch CSW closes (i.e. becomes on) when the signal DS changes to anactive value, here 1. The antenna circuit ACT then switches into a“damped” operating mode in which its impedance is modified such that theantenna signal AS decreases very rapidly.

The counter CPT is triggered by the change to 0 of the signal MS(DTx).The counter then puts the signal DS to 1 and starts counting. When thecounter has counted a number NC of cycles of the clock signal CKs, itresets the signal DS to 0. The counting of the NC cycles of the clocksignal CKs thus defines the duration of a phase Pd of damping theantenna signal, of a duration Td. When the damping phase Pd is over, thecounter CPT resets the damping signal DS to 0 and the switch CSW returnsinto the open state.

FIGS. 2A to 2E show the operation of the circuit ND1. FIG. 2A representsthe signal CKs, FIG. 2B represents the signal MS(DTx), FIG. 2Crepresents the signal MSK, FIG. 2D represents the damping signal DS, andFIG. 2E represents the signal Slm, composed of bursts of the signal CKs.For the sake of simplicity, the frequency of the signal CKs is notrepresented to scale. In FIG. 2B, periods P1 of emitting the magneticfield (bursts of magnetic field) in which the signal MS(DTx) is on 1, ofa duration T1, separated by periods P2 of not emitting the magneticfield of a duration T2, in which the signal MS(DTx) is on 0, can bedistinguished. In practice, the modulation signal MS(DTx) can have anyother shape according to the data to be sent DTx, to the coding appliedto the data DTx, and to the manner in which it is generated (inparticular with or without a sub-carrier).

The bursts of the signal CKs, of a duration T1, are applied to theantenna circuit when the modulation signal MS(DTx) is equal to 1 (aninverse convention could be provided). The signal MSK is put to 0 whenthe signal MS(DTx) is equal to 1, and is kept on 0 during maskingperiods Pm of duration Tm greater than T1 but below T1+T2. During themasking periods Pm, the oscillator OSC1 does not receive the externalclock signal CKe and operates in free oscillation mode.

Again with reference to FIGS. 2A to 2E, each non-emitting period P2includes:

-   -   the damping phase Pd, of duration Td,    -   a restoration phase Pr, of duration Tr, and    -   a resynchronization phase Psyn, of duration Tsyn, during which        the oscillator circuit is in the synchronous oscillation mode.

FIG. 2D shows the damping phase Pd (DS=1, switch CSW closed). The signalDS changes to 1 immediately after the signal MS(DTx) has changed back to0, i.e. when the non-emitting period P2 starts, and changes back to 0before the signal MSK changes back to 1, i.e. before theresynchronization phase Psyn starts, this lapse of time defines theduration Tr of the restoration phase. Therefore, the restoration phasePr follows the damping phase Pd and enables the amplitude of the antennasignal AS to be restored in the antenna circuit ACT. This restoration is“clean” as the damping phase Pd has eliminated the self-oscillationphenomenon which can occur at the end of the period Pl.

In summary, the free oscillation period Pm includes the emitting periodPl, the damping phase Pd, and the restoration phase Pr, i.e.Tm=T1+Td+Tr. The duration Tr of the restoration phase is equal toT2−Td−Tsyn and the duration Tsyn of the resynchronization phase Psyn isequal to T1+T2−Tm.

FIGS. 3A to 3D show more precisely the resynchronization mechanism ofthe signal CKs relative to the signal CKe, between two periods P1 ofemitting the magnetic field FLD2. FIG. 3A represents the signal MSK andis identical to FIG. 2C. FIG. 3B represents the signal CKe and FIG. 3Drepresents the signal CKs. FIG. 3C represents an internal oscillationsignal CKi of the oscillator circuit OSC1. The signals Cki and CKs areidentical in frequency and only differ in their amplitude, that of thesignal CKs being reduced here to the logic level 1 or 0.

FIG. 3B shows that the external clock signal CKe has a phase that mayfluctuate during the free oscillation periods. Indeed, during theseperiods, the internal clock signal CKs is injected into the antennacircuit and is superimposed on the signal induced by the externalmagnetic field FLD1. Even though the signal CKs is synchronized quiteprecisely with the external clock signal CKe, the phase shift betweenthe induced signal and the injected signal may be sufficient to cause aphase fluctuation rendering the antenna signal AS unable to supply areliable external clock signal CKe. FIGS. 3C and 3D show however, thatthe phase of the internal clock signal CKs remains substantially stableduring the free oscillation periods Pm thanks to the resynchronizationphases Psyn.

FIG. 4 represents the maximum phase shift Dtmax (expressed in time lag)between the signal CKs and the signal CKe, at the time the signal CKs isgoing to be resynchronized on the signal CKe, i.e. when the signal MSKis going to change back to 1. The value of Dtmax depends on theprecision of the oscillator circuit OSC1 that is chosen depending on theintended application. A maximum phase shift of ¼ of the period of thesignal CKe may for example be sought.

Thanks to the resynchronization phases Psyn occurring before eachemission of a burst of magnetic field FLD2, it is not necessary for theoscillator circuit OSC1 to have extreme precision guaranteeing a maximumphase shift of ¼ of the period over the entire duration of a data frameDTx. The necessary precision is reduced to the scale of one bit of dataDTx, and more precisely to the scale of the duration of a burst ofmagnetic field, which may represent only a fraction of a bit DTxdepending on the coding used (a binary datum may possibly be coded inseveral bursts of magnetic field).

FIGS. 5A to 5E schematically show a data receiving sequence DTr(left-hand side of the Figs.) as well as a data sending sequence DTx(right-hand side of the Figs.).

FIG. 5A shows the shape of the antenna signal AS. FIG. 5B represents themodulation signal MS(DTr) extracted from the antenna signal AS by thecircuit DMCT. FIG. 5C represents the active load modulation signal Slmincluding bursts of the signal CKs. FIG. 5D represents the modulationsignal MS(DTx) and FIG. 5E represents the signal CKs. The signal AS ishere a full-wave rectified ac signal that is the image of the magneticfield FLD1 in receive mode. Some embodiments of the antenna circuit ACTmay however include a rectifying diode for rectifying the antennasignal. In this case, the latter is a half-wave signal.

During the receipt of data DTr, the external device EDV (see e.g.,FIG. 1) modulates the amplitude of the magnetic field FLD1 with amodulation depth that depends on the chosen communication protocol, andthe antenna signal AS has a similar amplitude modulation. Periods ofnon-modulation NM in which the amplitude of the antenna signal ismaximum and modulation periods AM in which the amplitude of the antennasignal is minimum, can be distinguished. The maximum and minimumvoltages of the signal AS depend in practice on the distance between theexternal device EDV and the device ND1.

During the sending of data DTx, the antenna signal AS also hasnon-modulation periods corresponding to the periods P2 of not emittingthe magnetic field FLD2, during which its amplitude is identical to thatobserved during the non-modulation periods upon receiving data. Theamplitude of the antenna signal then only depends on the magnetic fieldFLD1 emitted by the external device EDV and on the distance between thetwo devices. The signal AS also has over-voltage periods correspondingto the periods P2 of emitting the magnetic field FLD2. Indeed, thesignal CKs is then injected into the antenna circuit and is superimposedon the signal induced by the external magnetic field FLD1. The antennasignal thus includes an induced component and an injected component.

FIG. 6 is a horizontally expanded view of the right-hand side part ofFIG. 5A and shows the decrease in the amplitude of the antenna signal ASduring the damping phase Pd and the increase in its amplitude during therestoration phase Pr (the amplitude of the antenna signal as representedis arbitrary). Depending on the damping rate provided when designing thedamping circuit, the amplitude of the antenna signal becomes low or zeroduring the period Pd, and is then restored to its normal value duringthe restoration phase Pr. The “normal” value of the antenna signal isthat induced by the external magnetic field FLD1. As indicated above,the damping phase enables a possible self-oscillation phenomenon to beneutralized after the sending period Pl, which could generate a spuriousantenna signal of a value greater than the value of this signal due tothe presence of the external magnetic field FLD1. If thisself-oscillation signal were not removed, the synchronous oscillator SOcould resynchronize on the self-oscillation signal instead ofresynchronizing on the external magnetic field FLD1.

Various types of oscillator circuits may be provided to produce thesynchronous oscillator SO. As an example, FIG. 7 shows an oscillatorcircuit OSC11 including an oscillator core of the astable multivibratortype (also referred to as an astable oscillator). This oscillator coreincludes two self-inductances L1, L2, a capacitor C1, two MOS-typetransistors T1, T2, and a source of current CG1. The inductance L1 isconnected between a supply node receiving a voltage Vcc and a node N1.The inductance L2 is connected between the voltage Vcc supply node and anode N2. The capacitor C1 is connected between the nodes N1, N2. Thetransistor T1 has its source S connected to the node N1, its drain Dlinked to the ground through the source of current CG1, and its gate Gconnected to the node N2. The transistor T2 has its source S connectedto the node N2, its drain D linked to the ground through the source ofcurrent CG1, and its gate G connected to the node N1.

Moreover, the synchronization input of the oscillator circuit OSC11 isformed by the base B of a bipolar transistor T3 the emitter E of whichis linked to the ground and the collector C of which is linked to thenode N1. The output of the oscillator circuit is formed by an invertergate I1 supplying the signal CKs, the input of which is linked to thenode N1, on which the internal oscillation signal CKi is present.

When the signal CKe is applied to the base B of the transistor T3(MSK=1), the phase of the signal CKi on the node N1 is imposed by thesignal CKe. The core of the oscillator circuit then no longer functionslike an astable oscillator but like a monostable oscillator.

When the signal MSK is equal to 0, the transistor T3 is no longerconductive and the core of the oscillator circuit functions in astableoscillator mode and oscillates freely at a self-oscillation frequency.This frequency is set as close as possible to the frequency of thesignal CKe (13.56 MHz in the framework of the abovementioned standards)by an appropriate choice of the inductances L1, L2 and of the capacitorC1.

In one alternative embodiment, the oscillator circuit OSC1 can be of thedigital type and configured, in the synchronous oscillation mode, toanalyze and store the frequency of the signal CKe received at thesynchronization input, and to copy the signal CKe at its output. When itswitches into the free oscillation mode, the oscillator circuit rebuildsthe signal CKs from this stored value.

As another example of an embodiment, FIG. 8 shows an oscillator circuitOSC12 including a phase locked loop. The circuit OSC12 includes a gateG10 of exclusive OR type, an active low-pass filter FT1, avoltage-controlled oscillator (VCO), a follower SCT, and a gate G11 ofAND type. The VCO supplies the internal oscillation signal CKi. Thelatter is applied to the follower SCT, the output of which supplies theinternal clock signal CKs. The internal oscillation signal CKi is alsosent back to an input of the gate G10 through the gate G11, whichreceives the signal MSK at another input. Another input of the gate G10forms the synchronization input of the circuit OSC12 and is linked tothe output of the gate G1, which supplies the external clock signal CKewhen the signal MSK is equal to 1. The gate G10 forms a phase comparatorand its output is sent to a control input of the VCO through the activelow-pass filter FT1.

When the signal MSK is equal to 1, the gate G11 is on, the gate G10receives both the signal CKi and the signal CKe and supplies aphase-difference signal. After low-pass filtering, this differentialsignal forms a setpoint voltage of the VCO. The signal Cki is thus phasecontrolled with the signal CKe. When the signal MSK changes to 0, theoutputs of the gates G1 and G11 change to 0. The output of the gate G10is forced to 0. Means enable the setpoint voltage that the VCO receivedbefore the signal MSK changed to 0 to be maintained at the input of theVCO. These means include for example a voltage source arranged in theactive low-pass filter FT1, such as a charge pump. Thus, the VCOcontinues to supply the signal CKi and maintains the phase of the signalCKi close to that of the signal CKe.

It will be understood by those skilled in the art that the datasending/receiving device ND1 according to the present invention issusceptible of various embodiments. In particular, the modulationcircuit MCT may, in addition to the active load modulation means thathave been described, include conventional passive load modulation means,such as a load modulation switch connected to the antenna circuit ACT.

Similarly, the damping circuit DCT is susceptible of variousembodiments. For example, instead of linking a point of the antennacircuit to the ground, the circuit DCT can be designed to link twopoints of the antenna circuit that are not generally linked, or even tofully short-circuit the antenna circuit so as to totally neutralize theantenna signal. Generally speaking, the impedance of the antenna circuitcan be modified with an actual load (short-circuit, switched resistor inparallel with elements of the antenna circuit) or an imaginary load(i.e. inducing a phase shift of 90°), for example a capacitor or aself-inductance that modifies the resonance frequency of the antennacircuit, or a combination of both. The aim is to rapidly discharge theantenna coil to retrieve an antenna signal that is only generated by theexternal magnetic field and free from any self-oscillation component.

The number NC can be fixed or programmable, for example by way of awrite-accessible register of the counter CPT. The damping circuit canalso be programmable and offer several damping rates which can beselected when it is commissioned.

In one alternative embodiment, the signal MSK is applied to the gaincontrol circuit AGC and the latter is configured to force the output ofthe amplifier A2 to 0. In this case, it may not be necessary to preventthe application of the signal CKe to the synchronization input of theoscillator circuit OSC1, the latter automatically switching into thefree oscillation mode when the signal CKe changes to 0.

The device ND1 may also include means for extracting a supply voltageVcc from the antenna signal, such as a rectifier circuit followed by asmoothing capacitor, to enable the device ND1 to operate in a purelypassive mode in the absence of a local power source.

According to one embodiment represented in FIG. 9, the device ND1 alsoincludes an oscillator OSC2 that is not synchronized to the externalclock signal CKe, for example a quartz oscillator. The oscillator OSC2supplies a clock signal CK2 allowing the device DV1 to operate in NFCreader mode to dialogue with a passive contactless integrated circuit orwith a similar device configured in passive mode. In this case, thesynchronous oscillator circuit OSC1 may continue to be used to supplythe internal clock signal CKs. The clock signal CKe or CK2 applied tothe synchronization input of the circuit OSC1 may be selected by way ofthe signal DET described above, by giving priority to the passive mode.

Again with reference to FIG. 9, two gates G3 and G4 are added and thegate G1 is modified to have three inputs. The gate G3 is an AND gatehaving three inputs one of which is an inverting input. The gate G4 isan OR-type gate with two inputs. The gate G3 receives the signal CK2 andthe signal MSK at its two non-inverting inputs, and receives the signalDET at its inverting input. The gate G1 receives the signals CK2, DET,and CKe at its inputs. The outputs of the gates G1 and G3 are applied tothe gate G4, the output of which is linked to the synchronization inputof the oscillator circuit OSC1. When the signal DET is equal to 1(presence of an external magnetic field FLD1), the output of the gate G3is forced to 0 and the device operates in passive mode in the mannerdescribed above. When the signal DET is equal to 0, the output of thegate G1 is forced to 0 and the signal MSK is forced to 1 by the circuitMG. The clock signal CK2 is applied to the input of the oscillatorcircuit OSC1, for the permanent emission of the magnetic field FLD1. Thestructure of the gate G2, receiving the signals CKs and MS(DTx) andsupplying the signal Slm, may be modified so that the latter ensures amodulation of the signal CKs with an adjustable modulation depth lowerthan 100%, in particular if the device must comply with ISO 14443A and14443B standards.

In the example of an embodiment represented in FIG. 10, the device ND1further includes a phase shifter DPH. This phase shifter is for examplearranged at the output of the amplifier Al. The phase shifter DPHapplies a phase shift Dp to the antenna signal AS and supplies the clockcircuit CKCT and the demodulation circuit DMCT with a phase-shiftedantenna signal AS′. The phase shift Dp may be fixed and determined upondesigning the phase shifter DPH. Alternatively, the phase shift Dp maybe determined dynamically by a command PHC applied to the phase shifter,which specifies the value of the phase shift Dp, for example between 0and 360°.

The phase shifter DPH enables the device ND1 to emit bursts of magneticfield FLD2 that have the phase shift Dp relative to the externalmagnetic field FLD1. This enables for example, in certain applicationsin which the communication distance must not exceed a certain threshold(for security reasons for example), the communication distance to bereduced below this threshold.

This embodiment shows that controlling the phase of the signal CKs bymeans of the synchronous oscillator SO enables more than just a zerophase shift between the magnetic field FLD2 and the magnetic field FLD1to be targeted. Controlling the phase of the signal CKs by way of thesynchronous oscillator SO may also enable a voluntary desynchronizationof the magnetic field FLD2 relative to the magnetic field FLD1 to becontrolled, while maintaining the phase shift Dp between the twomagnetic fields at a value as constant as possible.

The data sending/receiving device ND1 according to the present inventionis also susceptible of various applications. In an example ofapplication represented in FIG. 11, the device ND1 is integrated into aportable device HD1 and is linked to one or more host processors, heretwo host processors HP1, HP2 that use the device ND1 as a contactlesscommunication interface (NFC interface). The portable device HD1 is forexample a mobile telephone, a digital music player, or a personaldigital assistant (PDA). The processor HP1 may be the main processor ofthe device, or a secure processor such as a SIM card processor. Theprocessor HP2 may for example be the baseband processor of a mobiletelephone, also ensuring GSM channel communications.

In another example of application represented in FIGS. 12A, 12B, thedevice ND1 is linked to a host processor HP1 and the assembly isintegrated into a plastic CD medium to form a smart card HD2. Theantenna coil AC1 is for example a coplanar coil having one or more turnson its rear face (FIG. 12B). The card HD2 is equipped with a group CP ofcontacts. The card HD2 may for example form an NFC SIM card. The groupof contacts may in this case include contacts C1 to C8 according to ISO7816 standard. The card HD2 may also form a card of SD type, intended tobe inserted into any type of device (mobile telephone, personalcomputer, or the like) as an NFC communication interface.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A method comprising: producing, by an inductive antenna circuit, afirst periodic signal that is based on an alternating external magneticfield; producing, by an oscillator circuit, a second periodic signalthat is based on the first periodic signal; transmitting, incorrespondence with a data-carrying modulation signal, a sequence ofdata bits of a data frame by sequentially and repetitively: applying,with the oscillator circuit operating in a free oscillation mode, thesecond periodic signal to the inductive antenna circuit to transmit aportion of the data frame, the second periodic signal being applied tothe inductive antenna circuit in response to the data-carryingmodulation signal having a first logic value; after transmitting theportion of the data frame, inhibiting application of the second periodicsignal to the inductive antenna circuit, application of the secondperiodic signal to the inductive antenna circuit being inhibited inresponse to the data-carrying modulation signal having a second logicvalue, the second logic value being opposite the first logic value; andduring at least a portion of a period of time that application of thesecond periodic signal to the inductive antenna circuit is inhibited,placing the oscillator circuit in a synchronous oscillation mode, theplacing the oscillator circuit in the synchronous oscillation moderesulting in the second periodic signal being synchronized to the firstperiodic signal.
 2. The method of claim 1, further comprising:inhibiting application of the first periodic signal to a synchronizationinput of the oscillator circuit when the oscillator circuit is placed inthe free oscillation mode; and applying the first periodic signal to thesynchronization input of the oscillator circuit when the oscillatorcircuit is placed in the synchronous oscillation mode, the applying andinhibiting application of the first periodic signal to thesynchronization input of the oscillator circuit including applying andinhibiting application of the first periodic signal to thesynchronization input of the oscillator circuit responsive to a maskingsignal.
 3. The method of claim 1, further comprising phase shifting thesecond periodic signal relative to the first periodic signal.
 4. Themethod of claim 1, wherein the sequence of data bits includes a sequenceof encoded data bits.
 5. The method of claim 1, further comprising:supplying a control voltage based on a phase signal; generating thesecond periodic signal responsive to the control voltage; and inhibitingsupplying of the phase signal when placing the oscillator circuit in thefree oscillation mode, so as to maintain a value of the control voltage.6. A device for sending a data frame, the device comprising: aninductive antenna circuit configured to produce a first periodic signalthat is based on an alternating external magnetic field; an oscillatorcircuit that is configured to produce a second periodic signal that isbased on the first periodic signal; the device being configured to:transmit, in correspondence with a data-carrying modulation signal, asequence of data bits of a data frame by sequentially and repetitively:applying, with the oscillator circuit operating in a free oscillationmode, the second periodic signal to the inductive antenna circuit totransmit a portion of the data frame, the second periodic signal beingapplied to the inductive antenna circuit in response to thedata-carrying modulation signal having a first logic value; aftertransmitting the portion of the data frame, inhibiting application ofthe second periodic signal to the inductive antenna circuit, applicationof the second periodic signal to the inductive antenna circuit beinginhibited in response to the data-carrying modulation signal having asecond logic value, the second logic value being opposite the firstlogic value; and during at least a portion of a period of time thatapplication of the second periodic signal to the inductive antennacircuit is inhibited, placing the oscillator circuit in a synchronousoscillation mode, the placing the oscillator circuit in the synchronousoscillation mode resulting in the second periodic signal beingsynchronized to the first periodic signal.
 7. The device of claim 6,wherein: application of the first periodic signal to a synchronizationinput of the oscillator circuit is inhibited when the oscillator circuitis placed in the free oscillation mode; and the first periodic signal isapplied to the synchronization input of the oscillator circuit when theoscillator circuit is placed in the synchronous oscillation mode, thedevice being configured to selectively apply and inhibit application ofthe first periodic signal to the synchronization input of the oscillatorcircuit responsive to a masking signal.
 8. The device of claim 6,wherein the oscillator circuit further includes a phase locked loopincluding: a phase comparator configured to provide a phase signal; anactive low-pass filter configured to receive the phase signal and supplya control voltage based on the phase signal; and a voltage-controlledoscillator configured to receive the control voltage and generate thesecond periodic signal responsive to the control voltage, the phasecomparator being inhibited from supplying the phase signal when theoscillator circuit is in the free oscillation mode, such that a value ofthe control voltage at an input of the voltage-controlled oscillator ismaintained.
 9. The device of claim 6, further comprising a phaseshifting circuit configured to phase shift the second periodic signalrelative to the first periodic signal.
 10. The device of claim 6,further comprising: at least one host processor configured to supply thedevice with data to be sent; an active load modulation circuitconfigured to provide the data-carrying modulation signal based on thedata to be sent.
 11. The device of claim 6, wherein the device ismounted in or on a portable medium.
 12. The device of claim 6, whereinthe device is included in a smart card.
 13. A method comprising:producing, by an inductive antenna circuit, a first periodic signal thatis based on an alternating external magnetic field; producing, by anoscillator circuit, a second periodic signal that is based on the firstperiodic signal; transmitting, in correspondence with a data-carryingmodulation signal, a sequence of data bits of a data frame bysequentially and repetitively: applying, with the oscillator circuitoperating in a free oscillation mode, the second periodic signal to theinductive antenna circuit to transmit a portion of the data frame, thesecond periodic signal being applied to the inductive antenna circuit inresponse to the data-carrying modulation signal having a first logicvalue; after transmitting the portion of the data frame, inhibitingapplication of the second periodic signal to the inductive antennacircuit, application of the second periodic signal to the inductiveantenna circuit being inhibited in response to the data-carryingmodulation signal having a second logic value, the second logic valuebeing opposite the first logic value; and a period of time afterinhibiting application of the second periodic signal to the inductiveantenna circuit and while continuting to inhibit application of thesecond periodic signal to the inductive antenna circuit, placing theoscillator circuit in a synchronous oscillation mode, the placing theoscillator circuit in the synchronous oscillation mode resulting in thesecond periodic signal being synchronized to the first periodic signal.14. The method of claim 13, further comprising: inhibiting applicationof the first periodic signal to a synchronization input of theoscillator circuit when the oscillator circuit is placed in the freeoscillation mode; and applying the first periodic signal to thesynchronization input of the oscillator circuit when the oscillatorcircuit is placed in the synchronous oscillation mode, the applying andinhibiting application of the first periodic signal to thesynchronization input of the oscillator circuit including applying andinhibiting application of the first periodic signal to thesynchronization input of the oscillator circuit responsive to a maskingsignal.
 15. The method of claim 13, further comprising phase shiftingthe second periodic signal relative to the first periodic signal. 16.The method of claim 13, wherein the sequence of data bits includes asequence of encoded data bits.
 17. The method of claim 13, furthercomprising: supplying a control voltage based on a phase signal;generating the second periodic signal responsive to the control voltage;and inhibiting supplying of the phase signal when placing the oscillatorcircuit in the free oscillation mode, so as to maintain a value of thecontrol voltage.
 18. A device for sending a data frame, the devicecomprising: an inductive antenna circuit configured to produce a firstperiodic signal that is based on an alternating external magnetic field;an oscillator circuit that is configured to produce a second periodicsignal that is based on the first periodic signal; the device beingconfigured to: transmit, in correspondence with a data-carryingmodulation signal, a sequence of data bits of a data frame bysequentially and repetitively: applying, with the oscillator circuitoperating in a free oscillation mode, the second periodic signal to theinductive antenna circuit to transmit a portion of the data frame, thesecond periodic signal being applied to the inductive antenna circuit inresponse to the data-carrying modulation signal having a first logicvalue; after transmitting the portion of the data frame, inhibitingapplication of the second periodic signal to the inductive antennacircuit, application of the second periodic signal to the inductiveantenna circuit being inhibited in response to the data-carryingmodulation signal having a second logic value, the second logic valuebeing opposite the first logic value; and a period of time afterinhibiting application of the second periodic signal to the inductiveantenna circuit and while continuting to inhibit application of thesecond periodic signal to the inductive antenna circuit, placing theoscillator circuit in a synchronous oscillation mode, the placing theoscillator circuit in the synchronous oscillation mode resulting in thesecond periodic signal being synchronized to the first periodic signal.19. The device of claim 18, wherein the oscillator circuit furtherincludes a phase locked loop including: a phase comparator configured toprovide a phase signal; an active low-pass filter configured to receivethe phase signal and supply a control voltage based on the phase signal;and a voltage-controlled oscillator configured to receive the controlvoltage and generate the second periodic signal responsive to thecontrol voltage, the phase comparator being inhibited from supplying thephase signal when the oscillator circuit is in the free oscillationmode, such that a value of the control voltage at an input of thevoltage-controlled oscillator is maintained.
 20. The device of claim 18,further comprising a phase shifting circuit configured to phase shiftthe second periodic signal relative to the first periodic signal. 21.The device of claim 18, wherein the data frame includes a plurality ofencoded data bits.